This invention relates to a WDM optical transmission system and an optical transmission line thereof.
Deterioration of transmission characteristics in an optical amplification repeating transmission system is mainly caused by waveform deterioration due to the synergistic effect of the chromatic dispersion characteristic and non-linear optical effect of an optical fiber which is a transmission medium.
An effective method for preventing the waveform deterioration owing to the chromatic dispersion characteristic is to periodically insert dispersion-compensating mediums having inverse chromatic dispersion to that of the optical transmission fiber at specific intervals. By using the above method, it is possible to keep the absolute value of accumulated chromatic dispersion under a specific level. Also, the nonlinear effect of the optical fiber can be reduced by enlarging the effective cross section of transmission fiber.
A wavelength division multiplexing (WDM) transmission system has attracted a great deal of public expectations as a means suitable for a higher-capacity transmission system. In the WDM transmission, a compensation dispersion value of a dispersion-compensating medium is generally set so as to compensate for the accumulated chromatic dispersion of a center signal with in a signal wavelength band. The wavelength in which compensation of its accumulated chromatic dispersion is optimized by the chromatic dispersion medium is to be called hereinafter an effective zero dispersion wavelength or a zero dispersion wavelength of the transmission system. In optical transmission systems in which the dispersion-compensating mediums are not inserted, the zero dispersion wavelength of the optical transmission fiber is identical to the effective zero dispersion wavelength.
When the WDM transmission is utilized in an optically amplified repeater transmission system, the amplification characteristic of a repeater amplifier is generally adjusted so as to flatten out limitlessly within a signal wavelength band. When optical power of each signal wavelength is set to be the same, S/N ratio and non-linear effect also become equivalent and, therefore, it has been considered easy to obtain the same transmission characteristic.
An example that improved transmission characteristics using the above-mentioned dispersion compensation and enlargement of effective cross section is described by Masatoshi SUZUKI et al in xe2x80x9c170 Gb/s transmission over 10850 km using Large Core Transmission Fiberxe2x80x9d, Postdeadline Paper, PD17, Optical Fiber Communication Conference, San Jose, U.S.A., February 1998.
Described in the reference are two kinds of test results of 10850 km transmission experiment, one is a transmission of thirty-two 5.3 Gb/s multiplexed wavelengths and the other is that of sixteen 10.6 Gb/s multiplexed wavelengths. The test results are shown in FIG. 19. The horizontal axis, the left vertical axis and the right vertical axis respectively represents wavelength, Q2 value and bit error rate (BER) corresponding to the Q2 value. Almost the same transmission characteristic is obtained at all wavelengths in the 5.3 Gb/s wavelength multiplexing transmission test, on the other hand, in the 10.6 Gb/s transmission, the transmission characteristic of the signal wavelengths apart from the effective zero dispersion wavelength deteriorates inversely proportional to the wavelength difference. This is because the accumulated chromatic dispersion of the wavelengths apart from the effective zero dispersion wavelength becomes excessive due to the influence of the dispersion slope of the optical fiber and in addition the nonlinear optical effect took part in causing the irreversible waveform deterioration.
As explained above, in the conventional WDM optical transmission systems, when the transmission rate becomes 10 Gb/s or more, it was difficult to obtain desired satisfactory transmission characteristics of the wavelengths having the large accumulated chromatic dispersion owing to the synergistic effect of the accumulated chromatic dispersion and nonlinear effect.
An object of the present invention is to provide a WDM optical transmission system and optical transmission line capable of realizing almost the same transmission characteristic in all wavelengths even at the transmission rate of 10 Gb/s or more.
In the invention, the power of wavelength-division-multiplexed and transmitted signal light of a plurality of wavelengths is set to reduce proportional to the wavelength difference of each signal from its effective zero dispersion wavelength on an optical transmission line. Accordingly, the nonlinear effect of the wavelengths having large chromatic dispersion becomes small. As a result, the synergistic effect of the chromatic dispersion and nonlinear effect becomes equivalent in the whole signal light and therefore satisfactory transmission characteristics can be realized in the whole signal light.
Also, the transmission characteristic of the optical transmission line is set so that the transmissibility of the wavelengths reduces proportional to the difference in wavelength from the effective zero dispersion wavelength of the optical transmission line. By controlling the power deviation of the signal light within a predetermined value, for instance several dB, the bad influence due to the low S/N ratio can be neglected. To realize the above configuration, it is required to dispose an optical amplifier having a gain characteristic corresponding to the transmission characteristic and/or a transmission characteristic adjuster corresponding to the transmission characteristic.
An optical transmitter can take either way of outputting each optical signal toward the optical transmission line at the same power or outputting a plurality of optical signals toward the optical transmission line in a convex-shape power spectrum distribution in which the power of the signal light reduces proportional to the difference in wavelength from the effective zero dispersion wavelength of the optical transmission line. Further, the optical transmitter, at the beginning, can output the signal light toward the optical transmission line in a concave-shaped power spectrum distribution in which the power of the signal light increases directly proportional to the difference in wavelength from the effective zero dispersion wavelength of the optical transmission line. In that case, the power deviation should be controlled within the level in which the power spectrum distribution of the signal lights changes from the convex-shaped to the at an early stage on the optical transmission line and the signal light propagates in the convex-shaped power spectrum distribution on most of the optical transmission line.
It is preferable that the optical transmitter comprises a control light generator for generating gain-shape control light having the wavelengths except for the ones in the wavelength band of the signal light and the gain-shape control light is selectively multiplexed with the signal light to be output toward the optical transmission line. Owing to the gain-shape control light, the transmission characteristic of the optical transmission line becomes controllable and also it becomes easy to deal with a variety of characteristics and variations with time of respective optical components. Therefore, the satisfactory transmission characteristic can be maintained for a long time.